PSYCH 202: Hemispheric Asymmetry & Split Brains

Hemispheric Lateralisation & the Left Brain/Right Brain Myth Hemispheric Lateralisation

The tendency for some cognitive functions to be more strongly associated with one cerebral hemisphere than the other

• Both hemispheres contribute to most behaviours, but they often make different contributions

Hemispheric Lateralisation & the Left Brain/Right Brain Myth What is the traditional left brain/right brain myth

The myth suggests

• Left hemisphere = logical, analytical, verbal, and focused

• Right hemisphere = creative, emotional, visual, and holistic

Modern neuroscience shows this is an oversimplification

Hemispheric Lateralisation & the Left Brain/Right Brain Myth Why is the left brain/right brain myth inaccurate

Most cognitive functions involve networks spanning both hemispheres

• while some functions are lateralised (e.g., language), neither hemisphere works independently in normal cognition

Hemispheric Lateralisation & the Left Brain/Right Brain Myth What did Johanne Muller’s Doctrine of Specific Nerve Energies suggest about perception

Our experience of the world depends on the activity of sensory nerves, not direct access to objective reality

• Perception is constructed by the nervous system

Historic Evidence for Hemispheric Asymmetries What did Broca discover?

Damage to the left inferior frontal lobe causes deficits in language production (Broca’s aphasia)

Historic Evidence for Hemispheric Asymmetries Broca’s Aphasia symptoms

Impaired speech production with relatively preserved comprehension

Historic Evidence for Hemispheric Asymmetries What did Wernicke discover?

Damage to the left TPJ causes deficits in language comprehension

Historic Evidence for Hemispheric Asymmetries Wernicke’s Aphasia symptoms

Fluent but semantically empty speech and impaired comprehension

Historic Evidence for Hemispheric Asymmetries Why were Broca’s and Wernicke’s findings important

They provided strong evidence that language functions are predominantly lateralised to the left hemisphere

Historic Evidence for Hemispheric Asymmetries What contribution did John Hughlings Jackson make to understanding lateralisation

He described patients with deficits in recognising people, objects, and places (“imperception”) and suggested the right hemisphere plays an important role in perception

Historic Evidence for Hemispheric Asymmetries According to Jackson, how do the hemispheres differ

• Left hemisphere = language and frontal executive functions

• Right hemisphere = perception and visual processing

Handedness and Contralateral Control What is the most obvious asymmetry in humans

Handedness

Handedness and Contralateral Control What is contralateral control

Each hemisphere control movement on the opposite side of the body

Handedness and Contralateral Control How is contralateral control achieved

Motor fibres in the corticospinal tract cross (decussate) before reaching the spinal cord

Handedness and Contralateral Control Which hemisphere controls the left hand

The right hemisphere

Mirror Neurons and Language Mirror Neurons

Neurons that fire both when an individual performs an action and when they observe another individual performing the same action

Mirror Neurons and Language where were mirror neurons first discovered

Area F5 of the monkey premotor cortex

Mirror Neurons and Language What evidence demonstrated mirror neurons

The same F5 neurons fired when a monkey performed a precision grip and when it watched an experimenter perform the same grip

Mirror Neurons and Language What is the human homologue of monkey area F5

Broca’s area

Mirror Neurons and Language What is the proposed relationship between neurons and language

Neural systems originally involved in action understanding and motor planning may have been co-opted during evolution for language functions

Mirror Neurons and Language What functions overlap within the human mirror neuron network

Language, action perception, imitation, visuomotor processing, and fine motor control

Mirror Neurons and Language What does the mirror neuron theory suggest about language evolution

Understanding actions and understanding language may rely on similar neural mechanisms

Split-Brain Research What is Split-Brain Operation

Surgical severing of the corpus callosum (callosotomy) to prevent severe epileptic seizures from spreading between hemispheres

Split-Brain Research Why is the corpus callosum important

It is the major fibre tract connecting the two hemispheres, allowing communication between them

Split-Brain Research What happens after a callosotomy

The hemispheres can no longer communicate effectively at the cortical level

Split-Brain Research What did split-brain research reveal about hemispheric specialisation

Each hemisphere can process information independently and possesses unique strengths

Split-Brain Research How does visual information each the hemispheres

• The left visual field projects to the right hemisphere

• The right visual field projects to the left hemisphere

Split Brain Verbal Report Study What happens when a word is presented to the right visual field of a split brain patient

The left hemisphere receives the information and the patient can verbally identify the word

Split Brain Verbal Report Study What happens when a word is presented to the left visual field of a split brain patient

The right hemisphere receives the informaton, but the patient cannot verbally report it

• but was able to identify the word with their left hand (asked to feel around for the target object in a pool of objects)

Split Brain Verbal Report Study implications

• Left -Hemisphere = Language Dominance

• Right -Hemisphere = Comprehension

The Left Hemisphere Interpreter Study Method

• 2 images shown, fixation point

  • right hemisphere/left VF: snow scene

  • left hemisphere/right VF: chicken claw

• each hand must select a congruent image from a pool of images that matches the corresponding scene

The Left Hemisphere Interpreter Study Result

Split Brain patients:

• chooses both correct images

• but when asked why they picked the shovel (snow scene), they answered “to clean out the chicken shed”

The Left Hemisphere Interpreter Study Implications

• Patients confabulated: making up answers to explain behaviour after action has been performed

• Propensity to describe things linguistically comes with the to need to tell a consistent story to map on behaviour

• brain performs actions before we are aware of them

  • behaviour first, explanation later

Split Brain & Drawing Study Method

Asked right-handed split brain patient to replicate 3D drawings with each hand

Split Brain & Drawing Study Results

• left-handed drawing = 3d structure

• right-handed drawing = lacks 3D structure

Split Brain & Drawing Study implications

• deficit in perception of the stimulus

• Split Brain patients’ left hemisphere have impoverished perceptual understanding of the world

Visual intelligence

• The construction of a visual “world” through the operation of perceptual inference

• The visual mechanisms (rules) available to the right hemisphere are more sophisticated than those available to the left

• Right hemisphere is more “visually intelligent” than the left

Split brain patients…

lack visual inference that allows them to infer things that are not present in the scene